/Engine/Source/Editor/EditorFramework/Public/Toolkits/IToolkitHost.h
Allows Toolkits to push widgets to the viewport. Passing a nullptr for the Viewport will add or remove the OverlayWidget to or from the ActiveViewport
virtual void AddViewportOverlayWidget ( [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > InOverlaidWidget, [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [IAssetViewport](API\Editor\UnrealEd\IAssetViewport) > InViewport )